qemu

mmap-alloc.c (8575B)

---

 /*
  * Support for RAM backed by mmaped host memory.
  *
  * Copyright (c) 2015 Red Hat, Inc.
  *
  * Authors:
  *  Michael S. Tsirkin <mst@redhat.com>
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or
  * later.  See the COPYING file in the top-level directory.
  */
 
 #ifdef CONFIG_LINUX
 #include <linux/mman.h>
 #else  /* !CONFIG_LINUX */
 #define MAP_SYNC              0x0
 #define MAP_SHARED_VALIDATE   0x0
 #endif /* CONFIG_LINUX */
 
 #include "qemu/osdep.h"
 #include "qemu/mmap-alloc.h"
 #include "qemu/host-utils.h"
 #include "qemu/cutils.h"
 #include "qemu/error-report.h"
 
 #define HUGETLBFS_MAGIC       0x958458f6
 
 #ifdef CONFIG_LINUX
 #include <sys/vfs.h>
 #endif
 
 size_t qemu_fd_getpagesize(int fd)
 {
 #ifdef CONFIG_LINUX
     struct statfs fs;
     int ret;
 
     if (fd != -1) {
         do {
             ret = fstatfs(fd, &fs);
         } while (ret != 0 && errno == EINTR);
 
         if (ret == 0 && fs.f_type == HUGETLBFS_MAGIC) {
             return fs.f_bsize;
         }
     }
 #ifdef __sparc__
     /* SPARC Linux needs greater alignment than the pagesize */
     return QEMU_VMALLOC_ALIGN;
 #endif
 #endif
 
     return qemu_real_host_page_size();
 }
 
 #define OVERCOMMIT_MEMORY_PATH "/proc/sys/vm/overcommit_memory"
 static bool map_noreserve_effective(int fd, uint32_t qemu_map_flags)
 {
 #if defined(__linux__)
     const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
     const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
     gchar *content = NULL;
     const char *endptr;
     unsigned int tmp;
 
     /*
      * hugeltb accounting is different than ordinary swap reservation:
      * a) Hugetlb pages from the pool are reserved for both private and
      *    shared mappings. For shared mappings, all mappers have to specify
      *    MAP_NORESERVE.
      * b) MAP_NORESERVE is not affected by /proc/sys/vm/overcommit_memory.
      */
     if (qemu_fd_getpagesize(fd) != qemu_real_host_page_size()) {
         return true;
     }
 
     /*
      * Accountable mappings in the kernel that can be affected by MAP_NORESEVE
      * are private writable mappings (see mm/mmap.c:accountable_mapping() in
      * Linux). For all shared or readonly mappings, MAP_NORESERVE is always
      * implicitly active -- no reservation; this includes shmem. The only
      * exception is shared anonymous memory, it is accounted like private
      * anonymous memory.
      */
     if (readonly || (shared && fd >= 0)) {
         return true;
     }
 
     /*
      * MAP_NORESERVE is globally ignored for applicable !hugetlb mappings when
      * memory overcommit is set to "never". Sparse memory regions aren't really
      * possible in this system configuration.
      *
      * Bail out now instead of silently committing way more memory than
      * currently desired by the user.
      */
     if (g_file_get_contents(OVERCOMMIT_MEMORY_PATH, &content, NULL, NULL) &&
         !qemu_strtoui(content, &endptr, 0, &tmp) &&
         (!endptr || *endptr == '\n')) {
         if (tmp == 2) {
             error_report("Skipping reservation of swap space is not supported:"
                          " \"" OVERCOMMIT_MEMORY_PATH "\" is \"2\"");
             return false;
         }
         return true;
     }
     /* this interface has been around since Linux 2.6 */
     error_report("Skipping reservation of swap space is not supported:"
                  " Could not read: \"" OVERCOMMIT_MEMORY_PATH "\"");
     return false;
 #endif
     /*
      * E.g., FreeBSD used to define MAP_NORESERVE, never implemented it,
      * and removed it a while ago.
      */
     error_report("Skipping reservation of swap space is not supported");
     return false;
 }
 
 /*
  * Reserve a new memory region of the requested size to be used for mapping
  * from the given fd (if any).
  */
 static void *mmap_reserve(size_t size, int fd)
 {
     int flags = MAP_PRIVATE;
 
 #if defined(__powerpc64__) && defined(__linux__)
     /*
      * On ppc64 mappings in the same segment (aka slice) must share the same
      * page size. Since we will be re-allocating part of this segment
      * from the supplied fd, we should make sure to use the same page size, to
      * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
      * avoid allocating backing store memory.
      * We do this unless we are using the system page size, in which case
      * anonymous memory is OK.
      */
     if (fd == -1 || qemu_fd_getpagesize(fd) == qemu_real_host_page_size()) {
         fd = -1;
         flags |= MAP_ANONYMOUS;
     } else {
         flags |= MAP_NORESERVE;
     }
 #else
     fd = -1;
     flags |= MAP_ANONYMOUS;
 #endif
 
     return mmap(0, size, PROT_NONE, flags, fd, 0);
 }
 
 /*
  * Activate memory in a reserved region from the given fd (if any), to make
  * it accessible.
  */
 static void *mmap_activate(void *ptr, size_t size, int fd,
                            uint32_t qemu_map_flags, off_t map_offset)
 {
     const bool noreserve = qemu_map_flags & QEMU_MAP_NORESERVE;
     const bool readonly = qemu_map_flags & QEMU_MAP_READONLY;
     const bool shared = qemu_map_flags & QEMU_MAP_SHARED;
     const bool sync = qemu_map_flags & QEMU_MAP_SYNC;
     const int prot = PROT_READ | (readonly ? 0 : PROT_WRITE);
     int map_sync_flags = 0;
     int flags = MAP_FIXED;
     void *activated_ptr;
 
     if (noreserve && !map_noreserve_effective(fd, qemu_map_flags)) {
         return MAP_FAILED;
     }
 
     flags |= fd == -1 ? MAP_ANONYMOUS : 0;
     flags |= shared ? MAP_SHARED : MAP_PRIVATE;
     flags |= noreserve ? MAP_NORESERVE : 0;
     if (shared && sync) {
         map_sync_flags = MAP_SYNC | MAP_SHARED_VALIDATE;
     }
 
     activated_ptr = mmap(ptr, size, prot, flags | map_sync_flags, fd,
                          map_offset);
     if (activated_ptr == MAP_FAILED && map_sync_flags) {
         if (errno == ENOTSUP) {
             char *proc_link = g_strdup_printf("/proc/self/fd/%d", fd);
             char *file_name = g_malloc0(PATH_MAX);
             int len = readlink(proc_link, file_name, PATH_MAX - 1);
 
             if (len < 0) {
                 len = 0;
             }
             file_name[len] = '\0';
             fprintf(stderr, "Warning: requesting persistence across crashes "
                     "for backend file %s failed. Proceeding without "
                     "persistence, data might become corrupted in case of host "
                     "crash.\n", file_name);
             g_free(proc_link);
             g_free(file_name);
             warn_report("Using non DAX backing file with 'pmem=on' option"
                         " is deprecated");
         }
         /*
          * If mmap failed with MAP_SHARED_VALIDATE | MAP_SYNC, we will try
          * again without these flags to handle backwards compatibility.
          */
         activated_ptr = mmap(ptr, size, prot, flags, fd, map_offset);
     }
     return activated_ptr;
 }
 
 static inline size_t mmap_guard_pagesize(int fd)
 {
 #if defined(__powerpc64__) && defined(__linux__)
     /* Mappings in the same segment must share the same page size */
     return qemu_fd_getpagesize(fd);
 #else
     return qemu_real_host_page_size();
 #endif
 }
 
 void *qemu_ram_mmap(int fd,
                     size_t size,
                     size_t align,
                     uint32_t qemu_map_flags,
                     off_t map_offset)
 {
     const size_t guard_pagesize = mmap_guard_pagesize(fd);
     size_t offset, total;
     void *ptr, *guardptr;
 
     /*
      * Note: this always allocates at least one extra page of virtual address
      * space, even if size is already aligned.
      */
     total = size + align;
 
     guardptr = mmap_reserve(total, fd);
     if (guardptr == MAP_FAILED) {
         return MAP_FAILED;
     }
 
     assert(is_power_of_2(align));
     /* Always align to host page size */
     assert(align >= guard_pagesize);
 
     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
 
     ptr = mmap_activate(guardptr + offset, size, fd, qemu_map_flags,
                         map_offset);
     if (ptr == MAP_FAILED) {
         munmap(guardptr, total);
         return MAP_FAILED;
     }
 
     if (offset > 0) {
         munmap(guardptr, offset);
     }
 
     /*
      * Leave a single PROT_NONE page allocated after the RAM block, to serve as
      * a guard page guarding against potential buffer overflows.
      */
     total -= offset;
     if (total > size + guard_pagesize) {
         munmap(ptr + size + guard_pagesize, total - size - guard_pagesize);
     }
 
     return ptr;
 }
 
 void qemu_ram_munmap(int fd, void *ptr, size_t size)
 {
     if (ptr) {
         /* Unmap both the RAM block and the guard page */
         munmap(ptr, size + mmap_guard_pagesize(fd));
     }
 }